Stop checking for 'btrfs filesystem label' support (!26)

btrfs-progs 3.12 includes 'btrfs filesystem label /dev/PTN NEWLABEL'
functionality so stop checking for this before enabling setting the
label.

    $ btrfs version
    Btrfs v3.12
    $ btrfs filesystem label --help
    usage: btrfs filesystem label [<device>|<mount_point>] [<newlabel>]

        Get or change the label of a filesystem

        With one argument, get the label of filesystem on <device>.
        If <newlabel> is passed, set the filesystem label on <newlabel>.

    $ echo $?
    0

Worst case scenario is that some how an old version of the btrfs command
is used which doesn't support the labelling functionality.  Then this
commit would change GParted from disallowing labelling of a btrfs, to
allowing it, but presumably it would fail with an error from the btrfs
command reporting so.  Arguably better from a support point of view.

Closes !26 - Remove support for btrfs-progs < 3.12
4 jobs from master in 21 minutes and 12 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #246394
centos_build

00:03:16

passed #246395
ubuntu_build

00:03:05

 
  Test
passed #246396
centos_test

00:07:05

passed #246397
ubuntu_test

00:17:55